A laser lighting device of related art used for laser annealing and other purposes uses a fly's eye lens that homogenizes the intensity of laser light in a plane perpendicular to the optical axis thereof. That is, a fly's eye lens is so formed as to have a substantially flat plate shape as a whole in which a plurality of convex lenses is arranged in a grid pattern. When laser light passes through a fly's eye lens, the light having passed through each of the convex lenses is focused at the focal point of the lens and then diverges. The plurality of divided laser light beams is superimposed on each other and incident on the following optical member. Therefore, even when light incident on the fly's eye lens, for example, has a non-uniform intensity distribution in a plane perpendicular to the optical axis of the incident light, the fly's eye lens can homogenize the non-uniform intensity distribution. For example, laser light emitted from a light source may have a Gaussian intensity distribution in a plane perpendicular to the optical axis of the laser light, or non-uniform illumination may occur when the laser light is reflected off a total reflection mirror or any other optical member in some cases. Providing a fly's eye lens can solve the undesirable situations described above.
When a fly's eye lens is used in a laser lighting device, however, the coherency of laser light undesirably produces interference fringes in an area irradiated with the laser light, resulting in non-uniform illumination.
A variety of technologies for reducing the degree of interference fringes produced by laser light having passed through a fly's eye lens have therefore been proposed. For example, patent document 1 describes that an optical path difference adjustment member for reducing the degree of laser light interference is provided between a beam expander that increases the width of laser light and a fly's eye lens.
Further, the inventors of the present application have proposed in patent document 2 a technology for reducing the degree of interference fringes by providing a first optical path difference adjustment member formed of a plurality of light transmissive members in the optical path of laser light emitted from a light source, causing light having passed through the first optical path difference adjustment member to pass through a fly's eye lens where the intensity of the laser light is homogenized, causing the homogenized laser light to pass through a condenser lens where the laser light is collimated, causing the collimated laser light to pass through a second optical path difference adjustment member formed of a plurality of plate-shaped light transmissive members, and causing the light having passed through the second optical path difference adjustment member to pass through another fly's eye lens.
Patent document 1: Japanese Patent Application Kokai Publication [No. 2004-12757]
Patent document 2: Japanese Patent Application Kokai Publication [No. 2010-182731]